Method of enameling



Feb. 23, 1937. J. ROWAN 2,071,765

METHOD OF ENAMEL ING Filed March 15, 1955 INVENTOR Patented Feb. 23, 1937 UNITED STATES PATENT OFFICE 4 Claims.

My invention relates to the art of coating metals and, specifically, to enameling.

Enamel coatings have been applied to metal surfaces heretofore by covering the surfaces with successive layers of coating material and fusing each layer as applied by a so-called burning operation. At least two coatings and two burnings were required, and more if it was desired that the finished article have parts of contrasting colors. This practice was expensive and slow and required large amounts of equipment and facilities. In addition, the product was not entirely satisfactory from the standpoint of durability of the enamel coating.

Previous attempts to enamel the surfaces of steel or iron articles by applying a single layer of material and firing it, resulted in the discoloration of the enamel coat, particularly where it was light in color. This discoloration apparently resulted from the migration of particlesof iron or iron oxide from the surfaces of the articles into the enamel coat. The migration of these particles not only discolored the enamel coat but had a deleterious effect on the quality thereof.

I have invented a method of producing enamel coatings of two or more contrasting colors on the surfaces of iron or steel articles, which involves only one firing or burning operation. The invention thus overcomes the objections to previous enameling practices and permits the production of a low cost enameled article in a variety of colors having a coating which is adherent and durable and lacking in any discoloration.

In accordance with my invention, I subject the article to be e nameled, usually a stamping made from sheet metal, to a thin superficial coating of a solution of a salt of a metal in the same periodic group as iron, viz., cobalt or nickel. Alternatively, I may use a spray of a frit composed largely of an oxide of such elements. The superficial coating is then dried and an enamel coating applied thereover. The enamel coating is likewise dried. An acid-resisting frit is then preferably applied over the enamel coat and after it has dried, the article is subjected to firing at a temperature of about 1600 F., for a period slightly longer than that required under present practice. The several coatings are thereby fused and caused to adhere securely to the steel base, without any migration of the material of one layer into adjacent layers.

Where articles of contrasting colors are to be made, the enamel layers forming the different colors are applied to the respective portions of the article in turn, and then simultaneously fired.

The operations involved in practicing the method of my invention to manufacture a specific article, viz., a washing machine tub, are illustrated in the accompanying drawing by way of specific example only. In the drawing:

Figure 1 is a partial elevation of theartiole with the initial layer of enamel applied thereto; Figure 2 is a similar view showing the article with the color coat applied entirely thereover;

Figure 3 is a similar viewshowing a speckled coating applied to the. article;

Figure 4 illustrates the removal of the final spreckled coating from a portion of the article Figure 5 shows the article as completed; and

Figure 6 is a partial sectional view to enlarged scale showing the steel base and the several enamel coatings applied thereto.

In practicing the method of my invention, I take the article to be enameled after it has been suitably pickled or otherwise prepared for treatment, and apply to it a superficial ground coat of an enameling material rich in cobalt oxide. This condition is illustrated in Figure 1. Where it is desired to utilize the ground coat to provide zones of contrasting color, it is applied more heavily at the portions to be colored, for example, the rim of a tub such as shown at Figure l.

The initial coat is then dried in a low temperature oven at a temperature of between 200 to 300 F. until all the moisture is removed from the coating.

-A color coat is next applied and, in the usual case, this will be of a lighter color than the initial coat. Alternatively, before applying the sec- 0nd coat, I may subject the article having the superficial initial coat applied thereto all over inside and out, to a temperature of between 800 and 1000 F. for a few seconds, sufficient to give a very light fusion or superficial filming of the initial coat preferably around the rim, in case of tubs as illustrated in the drawing.

The second or color coat-is dried in the same way as the initial coat. If any of the material forming the color coat is deposited on the rim, it is removed as shown in Figure 2, before or after drying.

A speckle finish of a'color different from that of the color coat is then applied, as shown in Figure 3. This coating is also dried and excess material wiped from the rim, as shown in Figure '4. The article is then ready for the firing or burning treatment. For this purpose, it is placed in an enameling oven maintained at a temperature of about l600 for a period of three to four minutes. A complete fusion of all the coats results and a highly attractive enamel coating is produced.

Figure 6 illustrates the steel wall of the article being enameled having applied thereto a relatively thin layer of the initial coat, a relatively thicker layer of 'the color coat, and a third layer constituting the speckle finish, which is also very thin.

The third layer applied, instead of being a speckle finish, may be an acid-resisting glaze and, if so, the resulting article is characterized by resistance to the corrosive action of acids. In any event, the enamel coating is tenaciously adherent to the steel base and is characterized by a high degree of durability.

My explanation for the successful result produced by the invention is that the initial layer of a salt or oxide of a metal in the same periodic group as iron, forms a particularly efficient bond between the steel base and the heavier second color coat. I believe this phenomenon is analogous to the alloying of metallic coatings such as tin or zinc on steel. The enamel itself is really a solid solution of various oxides which make a frit or glaze. There are certain other oxides in the enamel which form an interlacingbond between the enamel and the steel upon which it is coated. Since iron and cobalt are in the same periodic group, they and their oxides have similar characteristics.

Several, alternatives to the above described process may be practiced with equally successful results in the production of a thoroughly acid-- proof or acid-resisting enamel. According to one of these, the ware is given a very thin superficial coat of an enamel mix which is rich in cobalt oxide. This coating is dried and a regular coating of white enamel containing a relatively high proportion of tin oxide as an opacifier is applied. This in turn is dried and covered with a coat of highly acid-resisting frit which is almost a clear quartz glass surface. The last coating is also dried at a low temperature and the ware is then fired at a temperature between 1550 and 1600. The resulting enamel is very firmly adherent and also acid-resistant.

Instead of the white enamel and the acid-resisting coating, I may substitute a color coat and a speckle finish. Instead of the initial cobalt ground coat, I may employ a light nickel or cobalt solution applied by dipping. When this is driesl, the color coat and acid-resisting glaze are successively applied and dried before firing,

As a finish coat, I may employ aspeckle containing a crystallizing enamel, for example, mother-of-pearl, to obtain a final finish covering a large portion of the surface of the article.

am also able to produce articles having portions of contrasting colors, at a single firing. The

.enamel coating is very adherent, highly durable,

and of beautiful appearance. There is no discoloration due to migration of the materials constituting the different layers in the coating. Light colored articles can be produced very readily and numerous combinations of contrasting colors are available. Since only one burning op eration isinvolved, the cost of production is reduced because of the small amount of time, labor and facilities required.

While I have disclosed only a certain preferred practice of the invention, it will be apparent that changes in the operations described may be made without departing from the spirit of the invention as defined by the appended claims.

I claim:

1. In a method of enameling metal ware, the steps including applying thereto a coating of a frit composed largely of cobalt oxide, heating the ware so coated to a temperature below that required for enameling yet sufficient to cause a superficial filming of the frit, i. e., to between 800 and 1000 F. then applying a color coating of enamel and heating the ware so coated to the enameling temperature.

2. ,The method of claim 1 characterized by applying said frit more heavily to certain parts of the article on which it constitutes a color coat, than to other parts on which it serves merely as a ground coat.

3. The method of claim 1 characterized by the steps of drying said enamel coating and applying a finish coating thereover before heating to the enameling temperature.

4. In a method of making enamel ware the steps including applying a thin coat of enameling material rich in cobalt oxide, heating the ware so coated to a temperature below that required for enameling but sufficient to cause a superficial filming of said material, applying a coat of white enameling material having tin oxide therein, drying the last-mentioned coat, applying thereover a frit composed largely of quartz glass, drying the frit, and firing the ware at a temperature of about 1600 F.

CLARENCE J. RODMAN. 

